Notes

Favoured by new cultivation techniques, weeds are becoming more and more of a problem in tropical agriculture. Unfortunately, few effective methods to control them have yet to be developed other than the problematic use of herbicides.
While agronomic problems change from country to country, the proliferation of weeds remains a constant problem. For farmers throughout the world, weeds always seem to grow better than crops. The situation appears to be even worse in Africa where cultivation techniques introduced over the last two decades have greatly increased the spread of weeds.
Adventitious plants do not pose a major problem for traditional forms of shifting or extensive agriculture.
While such fields may be full of many different weeds, including woody species, the use of crop associations that maintain a continuous ground cover greatly restricts their growth.
After clearing the plot before planting, farmers pull out the worst weeds by hand. Although this is a demanding process, it does not pose too great a problem on the majority of farms that cover less than 2-3 hectares. After several years of cultivation, when weeding becomes too onerous and soil fertility begins to drop, farmers simply abandon the plot and move on to another. The long fallow period not only improves fertility but also reduces the weed problem: the most adventitious plants are gradually forced out by the perennials.
The combination of more intensive cultivation and more mechanization has disrupted this delicate balance in many parts of Africa, notably the Sahel. The scarcity of agricultural land, and the high cost of clearing operations needed for mechanized cultivation, have slowed the expansion of production areas. Fallows become shorter and shorter, and often disappear completely. Mechanization has also led to the expansion of farm size: those with draught animals average 10 ha while those with small tractors average 20 ha.
The result of these changes could have been predicted: the invasion of weeds has reached the point of being the most important factor limiting yields. While mechanization enables larger areas to be worked, it demands more intensive cultivation which often means only a marginal increase in production. Small farmers are particularly affected by the lack of labour for weeding which, to be effective, must take place early in the cycle in order to 'nip the problem in the bud'
Even when mechanized, weeding can be difficult if, as is often the case, the soil is too wet or the seeding lines have been poorly drawn. Manual weeding is thus needed to complete the task. But caught by a demanding planting schedule, small farmers are rarely able to maintain sufficient control over invading weeds. These problems are exacerbated by the changes brought about by shortening or elimination the fallow period. In practice, the longer a field has been cultivated, the more weeds it contains. The situation evolves from one where a large number (50 to 60) of different weeds proliferate to one where fewer species (15 to 20) dominate the same area. After a while, it is primarily small, fast-growing grasses that cover the fields. Furthermore, many of these plants are favoured by the fertilizers appIied to the crops.
Intensification compromized
Such an increase in the number of weeds increases the workload and reduces both the yield and quality of the crop. This seriously compromises the intensification programmes launched by ACP countries. The limited data available show that production losses can reach 30-40 % for plots that are poorly weeded.
Worse still, the seeds of such weeds can reduce the value of harvests to the point of making them unusable. In Cote d'lvoire, it only takes six seeds of the grass Rottboellia cochinchinensis per 400 gr of rice to rule it out as a seed crop. In humid areas, weeds are even more of a problem, especially when fallows have been shortened. Soil quality permitting, it is preferable from a weeding point of view to have no fallow rather than one of just two years. But this means that women, who provide most of the labour for food crops, must spend even more time weeding. Perennial plots also need attention. Considerable work is needed during the first years, when the young plants are not tall enough to compete with existing or faster-growing weeds.
Research perspectives
Faced with such unpredictable invasions of weeds, agricultural researchers have only been able to recommend short-term measures. For all intents and purposes, this has meant the application of herbicides, the only technique that can quickly limit the damage. Basic research on weeds and production systems is needed to find other control methods, but this cannot be done overnight.
Most research in this field has been done on cotton, which was the first crop to receive widespread herbicide treatment. The development of Ultra Low Volume (ULV) spraying techniques facilitated the spread of chemical treatments. Farmers quickly realized how valuable such products were. Despite its relatively high cost, a single herbicide application during sowing operations increases production more than enough to pay for it. Apart from its efficiency, ULV spraying considerably reduces labour requirements an important factor in its rapid acceptance by farmers.
As a result, the acreage of chemically weeded cotton has grown very fast. According to estimates of the Institute for Research on Cotton and Textile Fibres (IRCT), only 35 ha were chemically weeded in 1976 but herbicides are now applied to over 80,000 ha in West Africa alone. Cote d'lvoire, followed by Mali, Senegal and Cameroon, are the largest users of herbicides. For several years now, food crops such as maize and upland rice, but also groundnuts, millet and sorghum, are more and more likely to be weeded chemically. This has not only resulted from the interest of farmers in improving the production of their other crops but from a technical consideration: it is often hard to restrict herbicides to just one crop. Because of their selective nature, herbicides can effectively destroy certain weeds but have no effect on others. Those that are used on cotton, for example, can eliminate small grasses but the ecological niche thus liberated is quickly filled by other weeds which, in turn, may only be controlled by herbicides developed for use with other crops. A comprehensive weeding programme must involve a variety of applications and crops. Finally, some plants are resistant to almost all herbicides and, with the competition reduced, they flourish.
While undoubtedly efficient, herbicides are not miracle solutions. This is particularly true for small farmers who may be poorly served by extension services. Proper herbicide use requires initial training and good follow-up. There is also an economic barrier for subsistence farmers or those whose cash crops cannot cover the cost of expensive, imported herbicides. Many small farmers thus have only their hands and hoes to control the increasing menace posed by weeds. It is primarily for their benefit that other techniques must be found. Despite the importance of this problem, and the lack of knowledge in this field, research on weeds is still very limited. A comprehensive list of the common weeds of West Africa has yet to be developed. There is also an urgent need for detailed studies on the biology and ecology of weeds, particularly for those plants posing the greatest problems to farmers without the means to control them.
Sorne research advances
Some weeds are finally receiving the kind of attention they deserve. A
- European project on striga. bringing together the British Weed Research Division and the French Centre for International Cooperation in Agricultural Research for Development, is currently studying host-parasite relations in weeds. It hopes to find comprehensive means to control this omnipresent weed which causes serious damage in Africa (responsible for up to 70 % of grain losses in sorghum crops in the Sudan). Infestations can be so bad that farmers have to abandon their fields.
Both the OAU and CILSS are concerned and are financing programmes designed to gather information on the techniques to adopt.
The same is true for Laos weed, Chromolaena odorata, whose economic and social impact is considerable. Having long been ignored by researchers, it is only now receiving extensive attention. In Sri Lanka, the International Centre for Biological Control is experimenting with biological methods, notably defoliating caterpillars. Native to Southeast Asia, Laos weed was inadvertently introduced to Africa by researchers, with disastrous results. It now requires supplementary weeding for food crops and for industrial plantations of coffee, cocoa, bananas and rubber, for which the use of herbicides and special cultivation techniques has become obligatory. Finally, even in pastures (notably in Central Africa), the proliferation of this forage weed is such that it can be monitored by remote sensing satellites!
Other problem weeds also deserve attention. These include the indestructible Commelina benghalensis or Euphorbia heteriphylla which invade parts of Cote d'Ivoire despite herbicide applications; Tridax which comes straight back after weeding; and the grass Rottboellia cochinchinensis, which is tall enough for its kernels to mix with rice seeds.
Damage can be limited by modifying cultivation techniques, including the use of crop associations which can impede the spread and growth of weeds. By sowing spreading cowpea 40 days after maize, the soil remains covered until the harvest
Other crop associations, which have yet to be investigated, can no doubt contribute to the battle. Post-harvest efforts to eliminate weeds enable rapid sowing when the rains come, before weeds can re-establish. This requires the use of earlymaturing varieties that can be harvested before the soil becomes too hard to work. Fertilization must also be modified in order to discourage weeds, some of which respond better to fertilizers than others
In humid regions, the International Institute of Tropical Agriculture (IITA) recommends the use of a ground cover, preferably a spreading nitrogen-fixing plant, which enables maize to be sowed without having to work the soil. This technique not only sharply reduces the number of weeds but improves soil fertility. The ideal solution, of course, would be to find crop varieties that can successfully compete all by themselves with weeds. Further work also needs to be done on companion plantinq and the use of plants that secrete substances that are toxic to certain weeds but not to the host plant. Such biological control methods are difficult to implement but they should not be ignored, especially in the case of widely distributed plants which invade non-cultivated zones such as forests and plains.
There is no doubt about the potential of herbicides. Recent research on antidotes which can increase their selectivity for specific crops opens up new perspectives. Interdisciplinary research that also considers the economic and social aspects is needed to ensure that herbicides are used safely by all farmers as part of an Integrated Pest Management programme.
BIBLIOGRAPHY
'Lutte raisonnee contre les mauvaises herbes, methodes ameliorees'. Production vegetale et protection des plantes, No. 44. FAO, Rome
Ange, A. and P. Fontanel, 1987. La contrainte enherbement et sa gestion dans le sud Saloum au Senegal.
Seminaire MESRU/CIRAD, Montpellier. France